The present invention relates to processes and devices for the separation of air by cryogenic distillation. It is known to distill air in a double column comprising a medium-pressure column connected thermally to a low-pressure column which surmounts it.
The thermal connection between the two columns can be obtained by using two evaporators placed one above the other in the low-pressure column. The lower evaporator can be heated by means of a nitrogen flow withdrawn from the medium-pressure column and then compressed in a cold compressor, and the upper evaporator can be heated by a flow of medium-pressure nitrogen taken from the medium-pressure column without having been compressed upstream of the evaporator.
A cold compressor is a compressor having a cryogenic inlet temperature, a cryogenic temperature being less than −50° C.
The nitrogen compressed in the cold compressor has to be condensed in the lower evaporator of the low-pressure column. The fluid compressed under cold conditions thus arrives relatively warm in the evaporator, with a high ΔT, before beginning to condense: this means that even if the evaporator has a low temperature pinch, the ΔT at the hot end is relatively high.
If the evaporator/condenser malfunctions, in particular if a partial blockage or poor distribution occurs, there is a high risk of evaporation to dryness locally, which is harmful to the safety of the device due to the presence of impurities of CnHm type with an oxygen-rich fluid. This is all the more appreciable on a falling-film evaporator/condenser.
FR-A-2 930 329 describes a process according to the preamble to claim 1.